Remove WebCompositorSupport::createDisplayItemList(), nothing uses it.

public/platform/WebCompositorSupport.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 23 matching lines @@
 #include "WebScrollbar.h"
 #include "WebScrollbarThemePainter.h"
 
 namespace blink {
 
 class WebCompositorAnimationCurve;
 class WebCompositorAnimationPlayer;
 class WebCompositorAnimationTimeline;
 class WebContentLayer;
 class WebContentLayerClient;
-class WebDisplayItemList;
 class WebExternalTextureLayer;
 class WebExternalTextureLayerClient;
 class WebFilterAnimationCurve;
 class WebFilterOperations;
 class WebFloatAnimationCurve;
 class WebGraphicsContext3D;
 class WebImageLayer;
 class WebLayer;
 class WebScrollbarLayer;
 class WebScrollbarThemeGeometry;
@@ skipping 32 matching lines @@
     virtual WebTransformAnimationCurve* createTransformAnimationCurve() { return nullptr; }
 
     virtual WebTransformOperations* createTransformOperations() { return nullptr; }
 
     virtual WebFilterOperations* createFilterOperations() { return nullptr; }
 
     virtual WebCompositorAnimationPlayer* createAnimationPlayer() { return nullptr; }
 
     virtual WebCompositorAnimationTimeline* createAnimationTimeline() { return nullptr; }
 
-    virtual WebDisplayItemList* createDisplayItemList() { return nullptr; }
-
 protected:
     virtual ~WebCompositorSupport() { }
 };
 
 }
 
 #endif // WebCompositorSupport_h